This invention relates to an intaglio printing plate and the printing method, more specifically, to the intaglio printing plate that enables thick film printing on a surface of hard materials and the printing method by means of such printing plate.
The printing method applied so far to thick film printing includes screen printing and transfer printing by rubber sheet. By these methods, however, film thickness of image line, pattern accuracy and dimensional accuracy have not reached the satisfactory level yet. In screen printing, for example, a screen is stretched between frames and the screen meshes other than the image are broken manually or by a photographic method to make it into a printing plate, then printing ink is forced through the screen mesh by squeezing for printing. This method has such disadvantages as broken line of image pattern, disordered edge or unstable dimension accuracy due to forced squeezing, since the ink is transfered through the mesh. With transfer printing by rubber sheet, on the other hand, ink is transfered to the surface of an elastic rubber sheet then is transcribed again directly onto the surface to be printed. This method has such disadvantages as distortion of image line or low dimensional accuracy for wide area printings, because the transfer is made by the elastic pressure of the rubber sheet. While, offset printing has long been used popularly as a highly productive printing method because of simple plate making. This is applicable to the printing on paper in general and to thin film printing, but is hardly applied to thick film printing. The reason is that the mechanism does not allow the thick film printing without pinholes and without irregular reproduction. In the following, the reason is described referring FIG. 1.
FIG. 1 shows the principle of ink transfer mechanism in offset printing. Being as such, the plate cylinder, inking arrangement and others are omitted. The object to be printed 12 is impressed by the impression cylinder 11 and the blanket cylinder 13, and the ink 14 transfered from the plate cylinder (not illustrated) is transcribed onto the surface of the blanket cylinder 13 as being exposed. This applies equally to letterpress printing, intaglio printing and also lithographic printing. The ink 14 transferred onto the blanket cylinder 13 easily flows out when subjected to the printing pressure P.sub.1 at transcription. The effluent ink divides into three parts: the first part 15a is the ink held tentatively on the elastic blanket 13 to be turned into the correct transcript image, a part of which remains on the blanket; the second part 15b is the ink forced out of the image position along the boundary between the blanket 13 and the object 12 causing irregular reproduction; and the third part 15c is the ink absorbed and fixed into the object to be printed 12. The irregular reproduction is, therefore, resulted because the ink is subjected to the printing pressure P.sub.1 while it is exposed on the blanket cylinder 13. The thicker the printing film is required, the higher the ink height H transferred onto the blanket 13 is. Accordingly, the ink for irregular reproduction rises, and then the printing quality becomes lower. Furthermore, when the object to be printed is of non-absorbing nature such as metal or glass, the part of irregular reproduction will be proportionally larger because the part 15c is not absorbed into the object 12. To reduce such irregular reproduction of image line, various contrivances have been tried as to make the ink harder or to reduce over-all printing pressure by controlling printing speed. Such contrivances, however, are not favorable in that the printed matter has many pinholes and is blurred, since these are consequently to minimize the printing pressure.
As mentioned above, quality evaluation of printed matters contains the antinomy of the faithfulness to the original plate i.e. the problem of irregular reproduction and the thickness of image line i.e. the problem of blurring, so offset printing in which the ink is transcribed onto the blanket cylinder as being exposed is not suitable essentially for thick film printing.
For thick film printing, it is essential that the quantity of ink can be controlled freely depending on the purpose of printing. Screen process printing and intaglio printing are the methods that can realize this. The screen process printing, however, has a disadvantage as mentioned above, in which the printing accuracy is not stable due to the squeezing pressure. In the intaglio printing, on the other hand, it is impossible to print directly onto hard materials, such as metal or glass, since the printing plate of this is made of metallic hard material.
For thick film printing on hard materials, there is no other way but to employ an indirect method that requires transferring the ink from metallic intaglio printing plate once onto a rubber blanket, then transcribing it again onto the object to be printed. After all, this is the same principle as that of offset printing and still includes the antinomy of irregular reproduction and blurring.